The immune system can respond in two ways when exposed to an antigen. A positive response leads to differentiation and proliferation of T and B cells, antibody production, killer T cells and to immunologic memory. A negative response leads to suppression, inactivation, or silencing of specific lymphocytes and to tolerance. Tolerance can be defined as the failure of a host to mount an immune response against a specific antigen. Normally, an organism is tolerant of its own antigens.
Autoimmune diseases are thought to result from an uncontrolled immune response directed against self antigens. In patients with multiple sclerosis (MS), for example, there is evidence that this attack is against the white matter of the central nervous system and more particularly to white matter proteins. Ultimately, the myelin sheath surrounding the axons is destroyed. This can result in paralysis, sensory deficits and visual problems. MS is also characterized by a lymphocyte and mononuclear cell infiltrate in the brain.
Susceptibility genes for MS have not been clearly identified, although the major histocompatibility complex HLA, particularly the DR2 haplotype, has been implicated. [Barcellos L. F., et al., Brain, 125: 150-158 (2002); Barcellos L. F., et al., Am. J. Hum. Genet., 72: 710-716 (2003); Coraddu, F., et al., Neurogenetics, 2: 24-33 (1998); Jersild, C., Svejgaard, A., Fog, T., Lancet, 1: 1240-1241 (1972).]
Autoreactive myelin-specific T cells, however, have been isolated from MS patients, although T cells of the same specificity have been detected in normal individuals. [LaSalle, J. M., et al., J. Immunol., 147:774-780 (1991); LaSalle, J. M., et al., J. Exp. Med., 176:177-186 (1992), Correale, J., et al., Neurology, 45:1370-1378 (1995).] Myelin-specific activated CD4 T cells secreting inflammatory cytokines (Th1 cells) appear to play a significant role in orchestrating myelin destruction. [Hemmer, B., Archelos, J. J., Hartung, H. P., Nat. Rev. Neurosci., 3: 291-301 (2002); Prat, E., Martin, R. J., Rehabil. Res. Dev., 39: 187-199 (2002).]
Some of the therapies described herein are aimed at specifically silencing these myelin-specific activated CD4 T cells, so they no longer respond to myelin antigen. [Baker, D., Hankey, D. J., Gene Ther. 10: 844-853 (2003); Furlan, R., Pluchino, S., Martino, G., Curr. Opin. Neurol., 16: 385-392 (2003); Mathisen, P. M., Tuohy, V. K., J. Clin. Immunol., 20: 327-333 (2000); Seroogy, C. M., Fathman, C. G., Gene Ther., 7: 9-13 (2000); Tarner, I. H., et al., Ann. N. Y. Acad. Sci. 998: 512-519 (2003).] Presently, the myelin proteins thought to be the target of an immune response in MS include, but are not necessarily limited to, myelin basic protein (MBP), proteolipid protein (PLP), and myelin-oligodendrocyte glycoprotein (MOG). Individuals who do not mount an autoimmune response to self proteins are thought to have control over these responses and are believed to be “tolerant” of self antigens. The evidence, therefore, that MS is caused by pathogenic T cells is necessarily indirect, but the close resemblance of the characteristics of this disease compared to those of the murine model, experimental autoimmune encephalomyelitis (EAE), suggest that MS is indeed caused by an aberrant immune response mediated by T cells.
The murine experimental autoimmune encephalomyelitis (EAE) mouse model for MS displays many of the same histopathological and clinical characteristics as the relapsing remitting forms of MS. [Zamvil, S. S., et al., Ann Rev. Immunol., 8:579-621 (1990); Brown, A. M., McFarlin, D. E., Lab. Invest. 45: 278-284 (1981); Kuchroo, V. K., et al., Annu. Rev. Immunol. 20: 101-123 (2002); Zhang. J., et al., J. Exp. Med., 179: 973-984 (1994).] EAE can be induced in SJL mice by injection of mouse spinal cord homogenate (MSCH), MBP, PLP, synthetic peptides whose sequences correspond to the major encephalogenic epitopes of myelin basic protein, MBP 84-104, proteolipid protein, PLP 139-151, or by adoptive transfer of activated CD4+ TH1, but not TH2 cells specific for encephalogenic epitopes. For example, EAE was induced in female SJL/J mice that was mediated by CD4+ T cells specific for proteolipid protein (PLP) amino acids 139-151. [Sobel, R. A., Greer, J. M., Kuchroo, V. K., Neurochem. Res., 19: 915-921(1994); Tuohy, V. K., et al., J. Immunol., 142: 1523-1527(1989); Tuohy, V. K., et al., J. Neuroimmunol., 39: 67-74 (1992).] In subsequent relapses, T cells specific for other encephalogenic epitopes, such as myelin basic protein (MBP) amino acids 84-104, have also been demonstrated. [McRae, B. L., Vanderlugt, C. L., Dal Canto, M. C., Miller, S. D., J. Exp. Med., 182: 75-85 (1995); Vanderlugt, C. L., et al., J. Immunol. 164: 670-678 (2000).]
The course of EAE in mice closely resembles clinical manifestations and pathology of relapsing and remitting MS in humans. This model is well known in the art, it is used to explore autoimmune mechanisms, test immunomodulating drugs directed at MS, and is the accepted analog to human multiple sclerosis. The major encephalogenic epitopes of myelin-derived sequences in EAE, such as MBP, can also activate human T cells of several different haplotypes including HLA-DR2. [Martin, R., et al., J. Exp. Med., 173:19-24 (1992).] The experimental disease is characterized by a relapsing-remitting course (R-EAE) of neurological dysfunction, perivascular mononuclear infiltration and demyelination. The mechanism of CNS damage appears to be mediated by inflammatory cytokines which can activate additional monocytes and macrophages non-specifically. [Blalock, J. E., The Immunologist, 2:8-15 (1994).]
Although the initial attack in EAE can be induced by the administration of either T cells specific for MBP or for PLP, close examination of reactivities of T cells in the primary and subsequent relapses demonstrated the presence of T cells which interact with specificities other than the inducing epitopes. This expansion of encephalogenic epitopes is termed “determinant spreading” or “epitope spreading.” [Miller, S. D. and Karpus, W. J., Immunology Today, 15:356-361 (1994); Lehman, P. V., et al., Nature, 358:155-157 (1992); Jiang, H., et al., Science, 256:1213-1215 (1992); Tuohy, V. K., et al., Immunol. Rev., 164: 93-100 (1998); Vanderlugt, C. L. and Miller, S. D., Nat. Rev. Immunol. 2: 85-95 (2002).] Antigen specific treatment would therefore, be expected to be more effective when administered early in the course of the disease, before the onset of increasing epitope complexity and eventual non-specific inflammation.
A way to treat autoimmune disease is the use of immunotherapy that can restore tolerance without suppressing the entire immune system which can lead to complications such as infection, hemorrhage, and cancer. Drugs currently used to treat autoimmune diseases have only been partially effective. Many of these drugs are non-specific immunosuppressive agents, anti-inflammatory agents or drugs which can block cell proliferation or depress proinflammatory cytokines or immunocytotoxic drugs. [Goodin, D. S., et al., Neurology, 58: 169-178 (2002); Hohlfeld, R. and Wiendl, H., Ann. Neurol., 49: 281-284 (2001); Martin, R., et al., Nat. Immunol., 2: 785-788 (2001); Steinman, L., Curr. Opin. Immunol., 13: 597-600 (2001).] Currently, immunomodulatory agents, such as interferon β-1A and 1B and glatiramer acetate are used to treat MS. In general, these agents are only effective for a limited duration and are subject to significant complications.
Thus it is desirable to suppress the immune system in a more specific way to control the response to self-antigens and theoretically “cure” the disease without down-regulating the entire immune system. In particular, a therapeutic approach that can downregulate pathogenic T cells while leaving the immune response otherwise intact may be an ideal solution. [von Herrath, M. G. and Harrison, L. C., Nat. Rev. Immunol., 3: 223-232 (2003).] Several specific immunotherapies have been hypothesized and tested in recent years, many of which are impractical or do not work in humans. For example, high affinity peptides can be synthesized which interact with MHC class II molecules and prevent the binding of encephalogenic peptides, thereby preventing the activation of pathogenic T cells. [Franco, A. et al., The Immunologist, 2:97-102 (1994).] This approach is disadvantageous in that it is difficult to obtain effective concentrations of inhibitor peptides in vivo. [Ishioka, G. Y., et al., J. Immunol., 152:4310-4319 (1994).] In an alternate strategy, peptides that are analogs of encephalogenic sequences have been shown to antagonize the T cell receptors of antigen-specific T cells, rendering them unreactive, although the exact mechanism is at present unknown. [Jameson, S. C., el al., J. Exp. Med., 177:1541-1550 (1993); Karin, N., et al., J. Exp. Med., 180:2227-2237 (1994); Kuchroo, V. K., et al., J. Immunol., 153:3326-3336 (1994).] Oral administration of myelin has been tested and found to induce a state of immunological unresponsiveness thought to be mediated by the induction of suppressor T cell or of anergy. [Weiner, H. L., et al., Annu. Rev. Immunol., 12:809-837 (1994); Whitacre, C. C., et al., J. Immunol., 147:2155-2163 (1991); Khoury, S. J., et al., J. Exp. Med., 176:1355-1364 (1992).]
In recent years, a variety of gene therapy strategies have also been used in EAE in mice. [Baker, D., Hankey, D. J., Gene Ther. 10: 844-853 (2003); Furlan, R., et al., Curr. Opin. Neurol. 16: 385-392 (2003); Mathisen, P. M. and Tuohy, V. K., J. Clin. Immunol., 20: 327-333 (2000); Seroogy, C. M. and Fathman, C. G., Gene Ther., 7: 9-13 (2000); Tamer, I. H., et al. Ann. N. Y. Acad. Sci. 998: 512-519 (2003).] These strategies were designed to prevent EAE rather than cure it. One gene therapy strategy used plasmids encoding the IL-4 gene together with myelin antigen, the PLP (139-151) epitope or myelin oligodendrocyte glycoprotein (MOG), which have been shown to elicit either protection in the case of PLP or amelioration of established disease in the case of MOG. [Garren. H., et al., Immunity, 15: 15-22 (2001).] Another approach has been to genetically modify antigen-specific T cells to deliver immunoregulatory molecules. [Chen, L. Z., et al., Proc Natl Acad Sci USA, 95: 12516-12521 (1998); Costa, G. L., et al., J. Immunol., 167: 2379-2387 (2001); Mathisen, P. M., et al., J. Exp. Med. 186: 159-164 (1997); Shaw, M. K., et al., J. Exp. Med., 185: 1711-1714 (1997); Yin, L., et al., J. Immunol., 167: 6105-6112 (2001).] In yet another approach, B cells were transduced with a vector encoding PLP (100-154) as well as B cells expressing a MBP-Ig fusion protein were shown to ameliorate ongoing disease. [Chen, C. C., et al., Blood, 103: 4616-4618 (2004); Melo, M. E., et al., J. Immunol., 168: 4788-4795 (2002).] Yet, another strategy includes the direct injection of naked DNA encoding anti-inflammatory cytokines. [Baker, D. and Hankey, D. J., Gene Ther., 10: 844-853 (2003).]
None of these strategies, however, have been able to effect a “cure.” As stated earlier, the standard of care currently has patients treated early in the course of disease usually with immunomodulatory molecules. The two commonly used immunomodulatory molecules include a synthetic amino acid polymer COPAXONE® (Teva Neuroscience) and the cytokine, interferon-β, which is manufactured with varying degrees of glycosylation and is marketed under the names of BETASERON® (Berlex/Schering), AVONEX® (Biogen)and REBIF® (Serono). At best these drugs are 30% effective and their side effects can be very significant and result in cessation of treatment. Interferon-β can cause flu-like symptoms, depression and liver damage. Patients can also generate antibodies which neutralize the cytokine thereby negating its therapeutic effect. COPAXONE® can cause allergic reactions which again results in termination of treatment.
In February 2005, a monoclonal antibody to the integrin VLA-4, called TYSABRI®, (Biogen and Elan) was withdrawn from market because two patients receiving AVONEX® together with TYSABRI® died from progressive multifocal leukoencephalopathy, a rare demyelinating disease caused by JC virus. The FDA is considering returning this drug to market because it proved to be 65% effective. It seems unlikely, however, that this molecule will be widely prescribed due to the extreme side effects.
Further, the cost of interferon-beta is $10-14,000 per patient per year while the price of COPAXONE® is $12-13,000 per patient per year. Thus, improvements are needed to treat MS and other autoimmune disorders with an effective, immunospecific approach.